Heat-expandable microspheres which have a structure comprising a shell of thermoplastic resin and a blowing agent encapsulated therein are generally called heat-expandable microcapsules. Thermoplastic resins usually include vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic copolymers, and blowing agents mostly employed are hydrocarbons, such as isobutane and isopentane. (See U.S. Pat. No. 3,615,972)
Such heat-expandable microcapsules are processed into lightweight hollow particulates (heat-expanded microspheres) by heating and expanding. For example, a process of spraying a dispersion of heat-expandable microcapsules into hot air to simultaneously expand and dry the microcapsules has been proposed as a process for expanding heat-expandable microcapsules. (See Japanese Patent Document JP B 59-53290) The process, however, presents a problem, in that the deposition of aggregated microcapsules at the end of the spray used is highly likely.
For solving those problems described above, the inventors of the present invention have developed a process for producing hollow particulates wherein heat-expandable microcapsules are heated and expanded in dry hot gas flow, the residual amount of unexpanded raw material, i.e. the heat-expandable microcapsule, in the expanded microcapsule is lowered, and the generation of aggregated microspheres is minimized. (See PCT Patent Document WO 2005/049698).